The present invention relates to improved knives for use in the lumber industry and particularly to a replaceble knife for a profile cutter.
In lumber mills, a log is debarked and shaped for subsequent cutting into dimensional lumber by a profile cutter. U.S. Pat. No. 3,457,974, issued on July 29, 1969, to L. A. Mitten, is representative of such a machine which includes means for controllably feeding a log into the cutting area which has several cutting heads, each of which in turn includes a plurality of cutting discs having removably mounted thereon a cutter knife.
Cutter knives of the prior art include cutting edges integral with a base including a curved gullett for directing wood chips away from the knife edge for removal from the machine. The feed means are specifically designed to prevent the cutting heads from their tendency to pull the tree through the machine at the speed of the cutting heads.
Several problems exist with profile cutters primarily directed to the knives which as can be appreciated are subject to intensive forces during the wood clipping process. Typically, knives wear only for a short period of time, not infrequently five hours or less. The integral knife and base construction of the prior art thus requires frequent sharpening of the knife edges. Inasmuch as the knife and base with a chip directing gullett are integral, regrinding of the knife edge also requires the gullett to be reground to maintain the predetermined relationship therebetween for proper chip removal. The time required for grinding both the knife edges and the gullett is extensive requiring skilled craftsmen and specialized grinding apparatus costing several thousand dollars as well as the necessary custom holders for securing the cutters to the grinding apparatus.
In an effort to overcome some of the difficulties of these machines, the industry has made intensive, heretofore unsuccessful, efforts to provide a knife assembly by which a replaceable knife is separately attached to a base which includes the gullett such that the knife removal can be expedited and the grinding operation simplified. One such effort is represented by U.S. Pat. No. 3,817,305, issued on June 18, 1974, to Wallace E. Gibbs. The structure represented by this prior art, however, requires a special knife holder and a curved knife, both of which in turn require specialized cutting discs which are assembled to form the cutter head. Thus, conversion of the profile cutter requires in essence an entirely new head to incorporate the replaceable knife feature of this prior art patent. As can be appreciated, this is an expensive proposition, and the curved knife has a relatively thin construction and is subject to failure due to crushing forces encountered when a tree inadvertently slips in the machine.
The machine's log feeding assemblies unavoidably permit some slippage of the logs. It has commonly been believed that this slippage caused shearing forces on the knifes which tore them apart. This belief led to the construction of the prior art which either did not provide replaceable knifes or provide a complex replaceable knife designed to resist shearing forces. After considerable experimentation it was discovered that logs jam against the knifes, and the knifes in fact fail due to a tremendous compressive or crushing force. Under such forces, the knives can be broken or deformed causing an unbalance of the cutting head assembly which can result in excessive vibrations causing even further machinery wear. During a typical operation, the conventional machines must be shut down frequently for repairs and knife replacement. This discovery led to the improved design of the present invention.